1. Field of the Invention (Technical Field)
This invention is related to aircraft flight guidance and more particularly a novel method and apparatus for the entry into a refueling rendezvous orbit with minimum maneuvering, and without violating airspace boundaries around the orbit.
2. Background Art
A problem occurs when a tanker is entering a rendezvous orbit in preparation for meeting with another aircraft, a receiver, in order to conduct airborne refueling operations. Normally the orbit pattern is anchored to a point in the airspace that is designated specifically for the air refueling mission. As such, the military aircraft that are to conduct the mission are expected to remain in the designated area, and all other aircraft are to be excluded. The point to which the rendezvous orbit is anchored (called the air refueling control point) is normally at the boundary of the designated area. This can make it very difficult for aircrews to enter the orbit without violating the boundary, and going outside the designated refueling area. This is a considerable problem when the aircraft is returning to the rendezvous orbit from inside the designated refueling area.
Presently, the prior art procedure preferred by air traffic control (ATC) for in-flight tanker refueling, is for the tanker to enter the rendezvous orbit by flying to the control point, over-flying it, and immediately turning onto the rendezvous leg (outbound leg), as shown in FIG. 1a. Performing this combined maneuver can result in two potentially difficult turns in rapid succession. FIG. 1a shows the technique typically used by tanker crews when flying the maneuver manually. As shown in FIG. 1a, any overshoot when performing this maneuver results in violating the airspace boundary.
The only prior art system that performs automated returns to the rendezvous orbit is the Collins FMS-800 system installed on some military aircraft. The technique used by this system is shown in FIG. 1b. This system computes aircraft position from a global positioning system (GPS) and inertial navigation system (INS) measurements, but its entry strategy and methodology is entirely different from the present invention. This system has no explicit protection for violating the airspace boundary, and it does not return to the orbit at the control point, as desired by ATC. Instead, the Collins system causes the aircraft to intersect the midpoint of the leg (inbound or outbound) whose bearing is closest to the bearing from the aircraft to the orbit. This allows computational simplicity, and avoids the necessity to perform two turns in rapid succession. However, it results in significantly compromised performance by requiring unnecessary time in the orbit.
Another prior art method is to approach the inbound leg on a path nearly parallel to the adjacent airspace boundary, overfly the control point, and continue in the orbit. The navigation aid used to assist the pilot in this maneuver is the inertial navigation system with its point-to-point or alignment way point navigation capability (A, B, C and D) as shown in FIG. 1c. The crew constructs a path along the boundary that passes through the control point. The pilot then approaches this path from inside the airspace boundary, monitoring cross-track error with respect to this path. This technique also requires unnecessary maneuvering.
The major disadvantage of the aforementioned prior art techniques are that they all involve extra maneuvering that wastes time and fuel. If a rendezvous is to take place, the sooner that a tanker can get into the outbound leg of rendezvous orbit, the less time is wasted in making the hook-up with the receiver. Note that the final rendezvous maneuver is made from the outbound leg, extended until the proper range is reached to the receiver for a successful rendezvous. These prior art approaches have also been limited by computational capabilities. They have attempted to achieve an entry into the rendezvous orbit that could be implemented within the throughput capability of the available processors. Prior art automated systems utilize a technique that captures the midpoint of a leg, thereby avoiding the necessity to enter another turn immediately upon re-entering the orbit. They have been limited by an incomplete understanding of the mission needs of the air crews that were performing the mission. Prior art approaches have focused on getting into the orbit on a straight line segment, thereby separating the orbit turning maneuver from the orbit re-entry maneuver. Furthermore, they have not made over-flying the control point a priority, contrary to the desires of ATC.
The present invention satisfies all constraints of a refueling orbit maneuver without requiring the aircraft to execute a series of difficult maneuvers in close succession in order to enter the orbit, over-fly the control point, and immediately enter the outbound turn. It accomplishes this by anticipating all the required maneuvers and constraints. Thus, as it enters the orbit, it overflies the control point (if physically possible), with a bank angle that results in entering the outbound turn, all in one smooth continuous maneuver, and without violating the airspace boundary.
The preferred method for maneuvering an aircraft for entering into a holding pattern, the aircraft position being known and the location and orientation of the holding pattern being known comprises the steps of dividing an airspace around the holding pattern into at least three zones, determining a relative bearing between a line from the aircraft to a control point of the holding pattern and an inbound leg line through the control point of the holding pattern, selecting an entry zone from the at least three zones based on the determined relative bearing, providing a predetermined entry maneuver for the selected entry zone and executing the predetermined entry maneuver. The at least three zones preferably comprise a first zone comprising the relative bearing of an angle between xe2x88x92180 degrees to  less than 0 degrees, a second zone comprising the relative bearing of an angle between  greater than +90 degrees to  less than +180 degrees and a third zone comprising the relative bearing of an angle between 0 to +90 degrees. The second zone preferably further comprises a zone 2b comprising a portion of second zone that lies between the inbound leg and an outbound leg extended and a zone 2a comprising a remaining portion of the second zone not within zone 2b. The preferred step of providing a predetermined entry maneuver for zone 2a comprises initially flying toward the control point and turning to capture the outbound leg of the holding pattern. The preferred step of turning to capture the outbound leg of the holding pattern comprises initiating the turn after intersecting the outbound leg. The preferred step of providing a predetermined entry maneuver for zone 2b comprises initially flying toward the control point and initiating a turn to capture the outbound leg of the holding pattern at a distance from the control point equal to twice an aircraft""s worst case turning radius. The preferred step of providing a predetermined entry maneuver for zone 1 comprises flying over the control point and capturing the outbound leg of the holding pattern. The preferred step of flying over the control point comprises flying directly to and over the control point. The preferred step of providing a predetermined entry maneuver for zone 3 comprises flying to intersect tangentially semicircle with a radius equal to an aircraft""s worst-case turn radius which lies within zone 3 and which passes through the holding pattern control point tangential to the holding pattern inbound leg. The preferred step of providing a predetermined entry maneuver for the selected entry zone comprises automatic commands to an autopilot. An alternative step of providing a predetermined entry maneuver for the selected entry zone comprises a manual command displayed to a crew. An alternative step of turning to capture the outbound leg of the holding pattern comprises initiating the turn before intersecting the outbound leg. The preferred holding pattern comprises a refueling rendezvous orbit.
The preferred method for maneuvering an aircraft for entering into a holding pattern, the aircraft position being known and the location and orientation of the holding pattern being known comprises the steps of determining a relative bearing between a line from the aircraft to a control point of the holding pattern and an inbound leg line through the control point of the holding pattern, dividing an airspace around the holding pattern into at least three entry zones, the three entry zones comprising a first zone comprising the relative bearing of an angle between xe2x88x92180 degrees to  less than 0 degrees, a second zone comprising the relative bearing of an angle between  greater than +90 degrees to  less than +180 degrees, wherein second zone further comprises, zone 2b comprising a portion of second zone that lies between the inbound leg and an outbound leg extended and zone 2a comprising a remaining portion of the second zone not within zone 2b and a third zone comprising the relative bearing of an angle between 0 to +90 degrees, providing a preselected entry maneuver for each entry zone, wherein the entry maneuver for the first zone comprises flying over the control point and capturing the outbound leg of the holding pattern, the entry maneuver for 2a comprises initially flying toward the control point and turning to capture the outbound leg of the holding pattern, the entry maneuver for zone 2b comprises initially flying toward the control point and initiating a turn to capture the outbound leg of the holding pattern at a distance from the control point equal to twice an aircraft""s worst case turning radius and the entry maneuver for zone 3 comprises flying to intersect tangentially semicircle with a radius equal to an aircraft""s worst-case turn radius which lies within zone 3 and which passes through the holding pattern control point tangential to the holding pattern inbound leg, electing an entry zone base on the determined relative bearing and executing the preselected maneuver for the selected entry zone.
A primary object of the present invention is to provide a path to the refueling orbit without unnecessary or undesirable maneuvers.
Another object of the present invention is to provide selected entry strategies based on three zones for orbit entry.
A primary advantage of the present invention is that it uses xe2x80x9cstandardxe2x80x9d flight management leg types in a unique combination to produce the desired maneuver with minimum xe2x80x9cwastedxe2x80x9d motion of the aircraft.
Another advantage of the present invention is that it follows air traffic control (ATC) directions more closely than present systems.
Yet another advantage of the present invention is that it sets the tanker up for a rendezvous with a receiver in minimum time.
Other objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.